This invention relates generally to techniques for intermingling a disinfectant with wastewater to be treated to kill pathogens therein, and more particularly to an in-line disinfectant contactor adapted to enhance contact between the disinfectant and the water to effect a more rapid and efficient disinfection activity than has heretofore been possible.
Chlorination is widely used to purify water supplies. In practice, chlorine is introduced at a selected point in the water supply system, flow then taking place into a tank or through a region of flow which is sufficient for the chlorine to act effectively on the contaminants present in the water to produce a disinfecting action. The amount of chlorine added to the water is referred to as the "dosage," and is usually expressed as milligrams per liter (mg/l) or parts per million (ppm). The amount of chlorine used up or consumed by bacteria, algae, organic compounds and some inorganic substances, such as iron or manganese, is designated as the "demand."
Since many of the reactions with chlorine are not instantaneous, but require time to reach completion, chlorine demand is time-dependent. The amount of chlorine remaining in the water at the time of measurement is referred to as the "residual." Residual is therefore determined by the demand subtracted from the dosage. Inasmuch as chlorine demand is time-dependent, this dependency is likewise true of chlorine residual.
When chlorine dissolves in water, a mixture of hypochlorous and hydrochloric acids is formed. The hydrochloric acid always completely dissociates into hydrogen and chloride ions, whereas the hypochlorous acid only partially dissociates into hydrogen and hypochlorite ions as a function of the pH of the water. In either the hypochlorous or hypochlorite form, chlorine is called "free chlorine residual." Free chlorine residual has a highly effective killing power toward bacteria.
Should the chlorinated water contain ammonia or certain amino (nitrogen-based) compounds, as is invariably the case with sewage, then additional compounds, called chloramines, are created. Chloramines may occur almost instantaneously, depending mainly on water pH. Though several reactions are possible between hypochlorous acid and ammonia, chloramines collectively are referred to as "combined chlorine residual." This combined chlorine residual has a much lower bactericidal effect than free chlorine residual.
Domestic wastewater is typically high in ammonia, the ammonia resulting primarily from hydrolysis of urea. Almost all of the inorganic nitrogen formed in solutions that enter a waste treatment plant is normally in the least oxidized, ammonia form. In conventional secondary waste treatment, a portion of the ammonia will be completely nitrified to nitrite, some ammonia will be only partially nitrified to nitrite, and a portion will remain as ammonia.
When sufficiently high chlorine dosages are applied to waters containing ammonia, different reactions will occur, resulting in the destruction of the ammonia and the formation of free chlorine residual. Thus, for water containing a known amount of ammonia, if one starts with a chlorine dosage which is low, chloramines will be formed resulting in a combined chlorine residual whose bactericidal effect is relatively weak.
As the dosage is raised, the amount of combined chlorine residual produced also increases, until a peak is reached when all of the free ammonia is used up in the formation of chloramine. And as the dosage is elevated beyond the level at which the combined chlorine residual peaks, destruction of the chloramines, which are unstable, takes place until a breakpoint is reached indicating that chloramine destruction is at its maximum. At breakpoint, the first persistent appearance of free chlorine occurs. Thus by using a chlorine dosage sufficient to attain the breakpoint state, one is able to get rid of virtually all ammonia and most of the chloramines.
The virtues of chlorination have long been appreciated, but it is only recently that the hazards involved in excessive chlorination have been publicly recognized. In studies carried out in the chlorinated water supply of the city of New Orleans, it was found that the levels of chlorination were such as to release carcinogenic agents dangerous to the community. The results of this study are reported in the article by R. A. Harris, "The Implication of Cancer Causing Substances in Mississippi River Water," published by the Environmental Defense Fund, Washington, D.C., Nov. 6, 1974.
Shortly after this study appeared, Public Law 93-523 went into effect authorizing the EPA administrator to conduct a comprehensive study of public water supplies "to determine the nature, extent, sources of, and means of control of contamination by chemicals or other substances suspected of being carcinogens."
Subsequently, Jolley ("Chlorine-containing Organic Constituents in Chlorinated Effluents"--Journal of the Water Pollution Control Fed., 47:601-618 (1975)) reported the presence of forty-four chloro-organic compounds in a chlorinated secondary wastewater effluent.
May applications exist for chlorine in wastewater treatment facilities, such as for odor control of raw sewage and the control of hydrogen sulfide in sewers, but its most universal application lies in wastewater treatment facilities for the terminal disinfection of the treated plant effluent just before the effluent is discharged.
The formation of compounds suspected of being carcinogenic as a result of the reaction of chlorine with hydrocarbons in wastewater is by no means the only unwanted side effect caused by the traditional disinfection process, for chlorine residuals in wastewater give rise to an environment that is toxic to aquatic organisms. Though chlorine is a highly effective biocide for undesirable organisms, it is also deadly to fish and other forms of aquatic life and therefore has a deleterious impact on fresh water eco-systems.
In general, wastewater disinfection practice has heretofore largely disregarded these unwanted side effects, for this practice focused on the two factors thought to be of greatest significance in attaining adequate disinfection; namely, the residual of the disinfectant and its contact time with the sewage. This practice has brought about the use of massive doses of disinfectant in long serpentine channels serving to prolong contact time. While this produced the desired degree of disinfection, it also aggravated unwanted side effects.
In order to obtain adequate disinfection with minimal unwanted side effects, the now-recognized goal is to carry out rapid, intimate mixing of the chlorine solution with the wastewater stream in the shortest possible period. Ideally, the mixing time should be a fraction of a second. With a view to attaining this goal, a jet disinfection technique has been developed to accelerate the mixing activity. This technique is described in the Penberthy Jet Disinfection Technical Bulletin published in 1977 by the Pentech Division of Houdaille Industries, Inc. of Cedar Falls, Iowa.
In the jet disinfection technique, the influent to be treated is pumped into a jet nozzle to which a chlorine supply is coupled, the nozzle projecting the influent into a reactor tube into which the chlorine is drawn by induced vacuum. Because of the highly turbulent field existing within the reaction tube, the disinfectant is thoroughly dispersed throughout the entire effluent flow and for an instant subjects the bacteria and viruses to an acutely toxic environment. The rapid and intimate contact of disinfectant with the wastewater brings chlorine in its most potent form to react on the pathogens, thereby promoting rapid kills. With higher kills, the long detention time of a conventional system is markedly reduced.
The most reactive species of chlorine is molecular chlorine, this being available in either chlorine gas or in a highly concentrated solution of aqueous chlorine. By introducing the molecular chlorine to the disinfection site as quickly as possible, unwanted side effects do not have time to occur. The intimate contact brought about by the jet disinfection technique shortens the contact time and reduces chlorine usage.
The overall operating expense of a chlorine disinfection system may be broken down into the respective costs of chlorine usage, water usage and power usage. Water usage depends on the carrier water required to create the necessary chlorine-water solutions, and since water usage is a function of chlorine demand, the lower the required chlorine dosage, the less carrier water entailed.
In a typical jet disinfection installation, a sealed baffle is placed across the wastewater channel to direct all channel flow through a plurality of reactor tubes. A portion of the incoming wastewater flow is internally pumped into the jet nozzle associated with each reactor tube, the chlorine being carried into the jet by induced vacuum. Thus, in addition to the external pump requirements for the channel, each jet nozzle assembly requires its own internal pump, thereby adding substantially to the overall cost of the system and creating maintenance problems.
While the present invention will be described in connection with chlorine as a disinfectant for wastewater, it is to be understood that the invention is also applicable to other disinfecting agents such as chlorine dioxide, bromine chloride and ozone.